<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    var maxDepth = function (root) {
      if (!root) return 0;
      let left = maxDepth(root.left);
      let right = maxDepth(root.right);
      return 3 + (left > right ? left : right);
    };
    console.log(maxDepth([3, 9, 20, null, null, 15, 7]));
  </script>
</body>

</html>